EP3072922B1 - Particules de mousse en résine de polyéthylène, article en résine de polyéthylène moulé par expansion dans le moule, et procédés de production respectivement de ces produits - Google Patents

Particules de mousse en résine de polyéthylène, article en résine de polyéthylène moulé par expansion dans le moule, et procédés de production respectivement de ces produits Download PDF

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EP3072922B1
EP3072922B1 EP14864413.1A EP14864413A EP3072922B1 EP 3072922 B1 EP3072922 B1 EP 3072922B1 EP 14864413 A EP14864413 A EP 14864413A EP 3072922 B1 EP3072922 B1 EP 3072922B1
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polyethylene resin
foamed particles
particles
less
weight
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German (de)
English (en)
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EP3072922A4 (fr
EP3072922A1 (fr
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Akihiro Itoi
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Kaneka Corp
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Kaneka Corp
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/16—Making expandable particles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
    • B29B9/00—Making granules
    • B29B9/10—Making granules by moulding the material, i.e. treating it in the molten state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C35/00—Heating, cooling or curing, e.g. crosslinking or vulcanising; Apparatus therefor
    • B29C35/02—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould
    • B29C35/04—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould using liquids, gas or steam
    • B29C35/049—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould using liquids, gas or steam using steam or damp
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C35/00—Heating, cooling or curing, e.g. crosslinking or vulcanising; Apparatus therefor
    • B29C35/16—Cooling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
    • B29C44/34—Auxiliary operations
    • B29C44/3461—Making or treating expandable particles
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/0061—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof characterized by the use of several polymeric components
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/12—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a physical blowing agent
    • C08J9/122—Hydrogen, oxygen, CO2, nitrogen or noble gases
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/16—Making expandable particles
    • C08J9/18—Making expandable particles by impregnating polymer particles with the blowing agent
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/22—After-treatment of expandable particles; Forming foamed products
    • C08J9/228—Forming foamed products
    • C08J9/232—Forming foamed products by sintering expandable particles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C35/00—Heating, cooling or curing, e.g. crosslinking or vulcanising; Apparatus therefor
    • B29C35/16—Cooling
    • B29C2035/1616—Cooling using liquids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
    • B29K2023/04—Polymers of ethylene
    • B29K2023/06—PE, i.e. polyethylene
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
    • B29K2023/04—Polymers of ethylene
    • B29K2023/06—PE, i.e. polyethylene
    • B29K2023/0608—PE, i.e. polyethylene characterised by its density
    • B29K2023/0633—LDPE, i.e. low density polyethylene
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2023/00—Use of polyalkenes or derivatives thereof as moulding material
    • B29K2023/04—Polymers of ethylene
    • B29K2023/06—PE, i.e. polyethylene
    • B29K2023/0608—PE, i.e. polyethylene characterised by its density
    • B29K2023/065—HDPE, i.e. high density polyethylene
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
    • B29K2105/04—Condition, form or state of moulded material or of the material to be shaped cellular or porous
    • B29K2105/046—Condition, form or state of moulded material or of the material to be shaped cellular or porous with closed cells
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2105/00—Condition, form or state of moulded material or of the material to be shaped
    • B29K2105/04—Condition, form or state of moulded material or of the material to be shaped cellular or porous
    • B29K2105/048—Expandable particles, beads or granules
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2995/00—Properties of moulding materials, reinforcements, fillers, preformed parts or moulds
    • B29K2995/0037—Other properties
    • B29K2995/0092—Other properties hydrophilic
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/02—Foams characterised by the foaming process characterised by mechanical pre- or post-treatments
    • C08J2201/034—Post-expanding of foam beads or sheets
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/06—CO2, N2 or noble gases
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2205/00—Foams characterised by their properties
    • C08J2205/04—Foams characterised by their properties characterised by the foam pores
    • C08J2205/052—Closed cells, i.e. more than 50% of the pores are closed
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C08J2323/06—Polyethene
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2423/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2423/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2423/04—Homopolymers or copolymers of ethene
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2423/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2423/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2423/04—Homopolymers or copolymers of ethene
    • C08J2423/06—Polyethene

Definitions

  • the present invention relates to polyethylene resin foamed particles, and a polyethylene resin in-mold foam-molded article.
  • Polyethylene resin foam-molded articles are utilized in various uses as a cushioning packaging material or a thermally insulating material, because of their excellent flexibility and thermal insulation property.
  • an in-mold foam-molding method in which polyethylene resin particles are previously foam-molded (bead-foaming) with a foaming agent such as butane gas, the foamed particles are filled in a mold, and the particles are heat-fused by introducing a heat medium such as steam.
  • a foaming agent such as butane gas
  • a cross-linked polyethylene has been often used because a foamed body having a high expansion ratio and an excellent heat-resistance can be obtained therefrom.
  • Even by using a non-cross-linked polyethylene resin having a good recyclability it is proposed to produce a molded article having a good moldability (see Patent Documents 1 and 2).
  • Patent Documents 3 and 4 disclose inventions that a linear low-density polyethylene resin is subjected to a heat-treatment (annealing) to obtain a polyethylene resin having two endothermic peaks on a melting curve obtained by a differential scanning calorimetry (DSC), the peak difference thereof being set at a certain value or more, and that when the obtained polyethylene resin is used as a base resin, a mold-processing range of the obtained polyethylene resin foamed particles can be broadened.
  • annealing to obtain a polyethylene resin having two endothermic peaks on a melting curve obtained by a differential scanning calorimetry (DSC), the peak difference thereof being set at a certain value or more
  • the linear low-density polyethylene is a copolymer of ethylene with an ⁇ -olefin, and the larger the amount of the ⁇ -olefin which is a comonomer, the lower the melting temperature of the copolymer, and thus it is easily forms crystals having a low degree of crystallinity. For that reason, in order to broaden the peak difference on the melting curve by the heat-treatment, resins having a wide distribution of the comonomer amount are required.
  • Patent Document 4 it can be considered that the distribution of the comonomer amount is broadened by mixing polyethylene resins having a density different from each other.
  • Patent Document 3 discloses, as foamed particles using a polyethylene resin having a high rigidity, foamed particles using, as a base resin, a blended resin of a high-density polyethylene having a density of 0.940 g/cm 3 or more and a melt index (which may also be hereinafter referred to as "MI") of 0.01 to 0.5 g/10 minutes, and a linear low-density polyethylene resin having a density of 0.920 g/cm 3 or more and 0.940 g/cm 3 or less and an MI of 0.1 to 10 g/10 minutes.
  • MI melt index
  • Patent Document 3 it is necessary to blend a large amount of the high-density polyethylene having a low MI and poor fluidity, and thus there is concern over the limitation of an amount of a resin processed and the decreased productivity due to the increased energy necessary for kneading and the increased pressure of an extruder, when resin particles, which are a starting material of foamed particles, are granulated.
  • foamed particles in which a high-density polyethylene is blended cause some cases where the foamed particles are shrunk or the mold-processing range is narrowed, comparted to foamed particles having two endothermic peaks on a melting curve obtained by the differential scanning calorimetry and having a peak difference of a certain value or more.
  • Patent Documents 5 and 6 also disclose foamed particles using, as a base resin, a mixed resin of two or more kinds of polyethylene resins. According to these techniques, however, it is necessary to mix with a low-density polyethylene, produced by a high pressure method, having a low melting point and a low rigidity, and thus there is concern over the reduced heat-resistance and the reduced mechanical properties.
  • open cell ratio a ratio of open cells (hereinafter referred to as "open cell ratio") of a molded article is increased when a heating temperature is raised upon the mold-processing (in-mold foam-molding), and the appearance and physical properties of the molded article may sometimes be deteriorated.
  • the present invention aims at providing polyethylene resin foamed particles capable of producing a polyethylene resin in-mold foam-molded article having a broad mold-processing temperature range, and good appearance and physical properties.
  • the present invention also aims at providing polyethylene resin foamed particles which can be produced without impairing a productivity of resin particles which are a starting material for foamed particles, and which are capable of producing a polyethylene resin in-mold foam-molded article having a broad mold-processing temperature range, and good appearance and physical properties.
  • the present invention further aims at providing, even if the foamed particles have a narrow peak temperature difference between temperatures of two melting peaks of a melting peak on a low temperature side and a melting peak on a high temperature side on a DSC curve obtained by a differential scanning calorimetry (DSC), polyethylene resin foamed particles which can be produced without impairing a productivity of resin particles, which are a starting material for foamed particles, and which are capable of obtaining a polyethylene resin in-mold foam-molded article having a little increased open cell ratio of a molded article and good appearance, even if a heating temperature is raised on mold-processing.
  • DSC differential scanning calorimetry
  • the present invention furthermore aims at providing polyethylene resin foamed particles capable of producing a polyethylene resin in-mold foam-molded article which has a little increased open cell ratio thereof, and which has good appearance and physical properties, even if a heating temperature is raised on mold-processing.
  • the present inventors have made painstaking studies in order to solve the problems described above. As a result, they have found that when polyethylene resin foamed particles, obtained by foaming polyethylene resin particles having a storage elastic modulus of 900 Pa or more and 5000 Pa or less at an angular frequency of 1 rad/second and a storage elastic modulus of 100000 Pa or less at an angular frequency of 100 rad/second in a dynamic viscoelasticity measurement at 190°C, are used, then, even if a heating temperature is raised on mold-processing, an in-mold foam-molded article having a little increased open cell ratio thereof and having good appearance and dimensional stability can be obtained without impairing a productivity of the resin particles; and that, even if polyethylene resin foamed particles have two melting peaks of a melting peak on a low temperature side and a melting peak on a high temperature side on a DSC curve obtained by a differential scanning calorimetry (DSC), and a peak temperature difference of 11°C or lower, when polyethylene resin foam
  • the present invention is as follows:
  • an in-mold foam-molded article having a little increased open cell ratio in the molded article and having a good appearance can be obtained, even if a heating temperature is raised upon mold-processing.
  • foamed particles can be produced without impairing a productivity of resin particles which are a starting material of the foamed particles, and an in-mold foam-molded article having a little increased open cell ratio in the molded article and having a good appearance can be obtained even if a heating temperature is raised upon mold-processing.
  • foamed particles of the present invention even if the foamed particles have a narrow peak temperature difference between respective temperatures at a melting peak on a low temperature side and at a melting peak on a high temperature side on a DSC curve obtained by a differential scanning calorimetry (DSC), foamed particles can be produced without impairing a productivity of resin particles which are a starting material of the foamed particles, and an in-mold foam-molded article having a little increased open cell ratio in the molded article and having a good appearance can be obtained even if a heating temperature is raised upon mold-processing.
  • DSC differential scanning calorimetry
  • Preferable embodiments of polyethylene resin foamed particles for attaining the objective of the present invention may be exemplified by the following aspects.
  • the polyethylene resin foamed particles 1 of the present invention uses a linear polyethylene resin as a base resin.
  • the linear polyethylene resin in the present invention refers to a homopolymer obtained by polymerizing ethylene using a polymerization catalyst and the like, and copolymers of ethylene with a comonomer copolymerizable with ethylene (comonomer is a short chain branching), which may include high-density polyethylene, medium-density polyethylene, linear low-density polyethylene, mixed resins thereof, and the like.
  • ⁇ -olefins having 3 or more and 18 or less carbon atoms may be used.
  • examples thereof may include propene, 1-butene, 1-pentene, 1-hexene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 1-octene, and the like. They may be used alone or as a mixture of two or more kinds.
  • At least one resin selected from the group consisting of low-density polyethylene resins and cross-linked polyethylene resins may be blended with the linear polyethylene resin within a range in which uniformity of cells in the polyethylene resin foamed particles and mechanical properties of an obtained in-mold foam-molded article are not impaired.
  • polyethylene resin particles are produced first.
  • the polyethylene resin particles have a storage elastic modulus at an angular frequency of 1 rad/second of 900 Pa or more and 5000 Pa or less, preferably 1100 Pa or more and 4000 Pa or less, more preferably 1300 Pa or more and 3000 Pa or less, in the dynamic viscoelasticity measurement at 190°C.
  • the storage elastic modulus at an angular frequency of 1 rad/second is less than 900 Pa in the dynamic viscoelasticity measurement at 190°C
  • the open cell ratio tends to be increased when the processing temperature is raised upon the in-mold foam-molding.
  • it is more than 5000 Pa it is difficult to expand the foamed particles upon the in-mold foam-molding and a surface beauty and the dimensional stability tend to be deteriorated.
  • the storage elastic modulus at an angular frequency of 100 rad/second is 100000 Pa or less, preferably 95000 Pa or less, more preferably 90000 Pa or less, in the dynamic viscoelasticity measurement at 190°C.
  • the storage elastic modulus at an angular frequency of 100 rad/second is more than 100000 Pa in the dynamic viscoelasticity measurement at 190°C, a resin pressure upon extrusion and a torque value upon kneading tend to be increased when resin particles, which are a starting material of the foamed particles, are produced, and the productivity tends to be reduced because of mechanical limitations.
  • the storage elastic modulus at an angular frequency of 100 rad/second there is no lower limit in the preferably range of the storage elastic modulus at an angular frequency of 100 rad/second, in general, when the storage elastic modulus at 100 rad/second is decreased by decreasing a molecular weight or the like, the storage elastic modulus at 1 rad/second tends to be decreased. Accordingly, when the storage elastic modulus values are satisfied, the range thereof is automatically restricted.
  • the storage elastic modulus is measured by using a rotary rheometer, and after a flat plate resin sample is thoroughly pre-heated at 190°C, an angle of rotation is changed at a constant period, and a stress is detected when a strain is applied.
  • a cone plate type is used as a measuring tool, and the measurement is performed in a nitrogen atmosphere. The measurement is performed in an angular frequency range of 0.1 rad/second to 100 rad/second, and a storage elastic modulus and a loss elastic modulus are obtained at each angular frequency.
  • the storage elastic modulus values at an angular frequency of 1 rad/second and at an angular frequency of 100 rad/second are adopted.
  • Method for obtaining polyethylene resin particles having a storage elastic modulus of 900 Pa or more and 5000 Pa or less at an angular frequency of 1 rad/second and having a storage elastic modulus of 100000 Pa or less at an angular frequency of 100 rad/second in the dynamic viscoelasticity measurement at 190°C is not particularly limited, and may be exemplified by a method using, as the base resin, a mixed resin of a linear polyethylene resin (A) having a high melt index MI as a main component and a linear polyethylene resin (B) having a very low melt index MI.
  • the polyethylene resin particles above has a melt index MI at 190°C of preferably 0.8 g/10 minutes or more and 3.0 g/10 minutes or less, more preferably 1.0 g/10 minutes or more and 2.5 g/10 minutes or less.
  • MI melt index
  • the storage elastic modulus is high, the storage elastic modulus at an angular frequency of 100 rad/second is very likely to exceed 100000 Pa, and the fluidity is worsened, and thus it tends to be difficult to obtain an in-mold foam-molded article having good appearance.
  • the storage elastic modulus at an angular frequency of 1 rad/second is likely to be less than 900 Pa, and an open cell ratio of the in-mold foam-molded article tends to be increased.
  • the MI of the polyethylene resin particles is a value measured at a temperature of 190°C with a load of 2.16 kg in accordance with JIS K 7210.
  • the polyethylene resin particles have preferably a density of 0.915 g/cm 3 or more and 0.940 g/cm 3 or less, more preferably 0.918 g/cm 3 or more and 0.935 g/cm 3 or less.
  • the polyethylene resin particles have a density of less than 0.915 g/cm 3 , it is concerned that the rigidity of the resin is low and the mechanical properties thereof are reduced.
  • the polyethylene resin particles have a density of more than 0.940 g/cm 3 , the resin may sometimes be brittle and it is concerned that the impact strength is reduced, and further the processing temperature range tends to be narrowed.
  • the linear polyethylene resin (A) having a high melt index MI as a main component is mixed with a linear polyethylene resin (B) having a very low melt index MI is used as the base resin of the polyethylene resin foamed particles 1, the linear polyethylene resin (A) having a high MI, which is the main component, has a melt index MI of preferably 1.2 g/10 minutes or more and 10 g/10 minutes or less, more preferably 1.2 g/10 minutes or more and 6.0 g/10 minutes or less.
  • the linear polyethylene resin (B) having a very low melt index MI has a melt index MI of preferably 0.01 g/10 minutes or more and 0.3 g/10 minutes or less, more preferably 0.02 g/10 minutes or more and 0.2 g/10 minutes or less.
  • the linear polyethylene resin (A) is contained in a content of 60% by weight or more and 97% by weight or less and the linear polyethylen resin (B) is contained in a content of 3% by weight or more and 40% by weight or less, to 100% by weight of the total of (A) and (B); and it is more preferable that the linear polyethylene resin (A) is contained in a content of 70% by weight or more and 95% by weight or less and the linear polyethylene resin (B) is contained in a content of 5% by weight or more and 30% by weight or less.
  • the storage elastic modulus at an angular frequency of 1 rad/second tends to be 900 Pa or more and 5000 Pa or less
  • the storage elastic modulus at an angular frequency of 100 rad/second tends to be 100000 Pa or less.
  • the linear polyethylene resin (A) has a density of preferably 0.915 g/cm 3 or more and 0.940 g/cm 3 or less, more preferably 0.918 g/cm 3 or more and 0.935 g/cm 3 or less.
  • the linear polyethylene resin (A) has a density of less than 0.915g/cm 3 , the rigidity of the base resin is low, and the reduction of the mechanical properties is concerned.
  • the linear polyethylene resin (A) has a density of more than 0.940 g/cm 3 , the base resin may sometimes be brittle and it is concerned that the impact strength is reduced, and further the processing temperature range tends to be narrowed.
  • the linear polyethylene resin (B) has a density of preferably 0.915 g/cm 3 or more and 0.970 g/cm 3 or less, more preferably 0.920 g/cm 3 or more and 0.970 g/cm 3 or less.
  • the linear polyethylene resin (B) has a density of less than 0.915 g/cm 3 , the rigidity of the base resin is low, and the reduction of the mechanical properties is concerned.
  • the linear polyethylene resin (B) has a density of more than 0.970 g/cm 3 , the base resin may sometimes be brittle and it is concerned that the impact strength is reduced.
  • it is difficult to melt the resin and it is concerned that the kneading property is reduced when the resin particles are produced, and that the obtained foam has an ununiform cell structure and reduced physical properties.
  • the ranges of the storage elastic modulus at an angular frequency of 1 rad/second and an angular frequency of 100 rad/second in the dynamic viscoelasticity measurement at 190°C and the reasons why they are preferable are the same as those in the polyethylene resin foamed particles 1.
  • a method for obtaining the polyethylene resin particles having a storage elastic modulus of 900 Pa or more and 5000 Pa or less at an angular frequency of 1 rad/second and a storage elastic modulus of 100000 Pa or less at an angular frequency of 100 rad/second in the dynamic viscoelasticity measurement at 190°C is not particularly limited, and may be exemplified a method using, as a base resin, a mixed resin of a polyethylene resin (A') having a high melt index as a main component with a polyethylene resin (B') having a very low melt index.
  • the polyethylene resin (A') has a melt index of preferably 1.2 g/10 minutes or more and 10 g/10 minutes or less, more preferably 1.2 g/10 minutes or more and 6.0 g/10 minutes or less.
  • the polyethylene resin (B') having a very low melt index has a melt index of preferably 0.01 g/10 minutes or more and 0.3 g/10 minutes or less, more preferably 0.02 g/10 minutes or more and 0.2 g/10 minutes or less.
  • the polyethylene resin (A') is contained in a content of 60% by weight or more and 97% by weight or less and the polyethylene resin (B') is contained in a content of 3% by weight or more and 40% by weight or less, to 100% by weight of the total of (A') and (B'); and it is more preferable that the polyethylene resin (A') is contained in a content of 70% by weight or more and 95% by weight or less and the polyethylene resin (B') is contained in a content of 5% by weight or more and 30% by weight or less.
  • the storage elastic modulus at an angular frequency of 1 rad/second tends to be 900 Pa or more and 5000 Pa or less, and the storage elastic modulus at an angular frequency of 100 rad/second tends to be 100000 Pa or less.
  • the ranges of the density of the polyethylene resins (A') and (B') and the reasons why they are preferable are the same as those in the polyethylene resins (A) and (B) in the polyethylene resin foamed particles 1.
  • the present invention also aims at obtaining foamed particles having a broad mold-processing range, such as foamed particles obtained from polyethylene resin particles having a high degree of crystallinity and a high rigidity, even if a polyethylene resin having a narrow melting temperature range is used as a starting material.
  • the polyethylene resin (A') has a density of 0.930 g/cm 3 or more, the polyethylene resin particles tends to have a crystal melting heat q of 145 J/g or more.
  • the polyethylene resin particles more preferably have a crystal melting heat q of 170 J/g or less.
  • the polyethylene resin has a crystal melting heat q of more than 170 J/g, the two melting peaks of the melting peak on a low temperature side and the melting peak on a high temperature side cannot be obtained on the DSC curve of the foamed particles, or the difference between the two melting peak temperatures is too narrow, and thus, the mold-processability may possibly worsened.
  • a method for producing the polyethylene resin particles 1 and 2 of the present invention may include, for example, a method described below.
  • polyethylene resins which are a starting material and additives (if necessary) are mixed in a dry-blending method, a masterbatch method, or the like.
  • the kneaded product is shredded into particles using a cutter, a pelletizer, or the like thereby obtaining polyethylene resin particles.
  • the polyethylene resin particles have a weight of preferably 0.2 mg or more and 10 mg or less per particle, more preferably 0.5 mg or more and 6.0 mg or less per particle.
  • the polyethylene resin particles have a weight of less than 0.2 mg per particle, the obtained in-mold foam-molded article may possibly have an increased shrinkage percentage.
  • it is more than 10 mg, it may be difficult to fill the particles in a mold.
  • the polyethylene resin particle weight per particle refers to an average resin particle weight obtained from 100 polyethylene resin particles, which are randomly selected.
  • the polyethylene resin particle weight per particle is hardly changed even after the foaming step, and thus there is no problem even if the polyethylene resin particle weight per particle is considered as the polyethylene resin foamed particle weight per particle.
  • the polyethylene resin particles of the present invention may contain, if necessary, a cell-nucleating agent, a hydrophilic compound, and additives such as an antioxidant, an anti-static agent, a coloring agent, and a flame retardant.
  • an inorganic nucleating agent such as talc, calcium stearate, calcium carbonate, silica, kaolin, titanium oxide, bentonite, or barium sulfate is generally used. They may be used alone or as a mixture of two or more kinds.
  • the talc is preferable among the cell-nucleating agents, because uniform cells can be obtained.
  • the hydrophilic compound it is preferable to add the hydrophilic compound to the polyethylene resin particles of the present invention.
  • the hydrophilic compound is added, the foamed particles having a high expansion ratio can be obtained, even if an inorganic gas is used as a foaming agent.
  • the hydrophilic compound used in the present invention refers to a compound which contains a hydrophilic group such as a carboxylic group, a hydroxyl group, an amino group, a sulfo group, or a polyoxyethylene group in its molecule and a derivative thereof, including a hydrophilic polymer.
  • a hydrophilic group such as a carboxylic group, a hydroxyl group, an amino group, a sulfo group, or a polyoxyethylene group in its molecule and a derivative thereof, including a hydrophilic polymer.
  • the compound containing a carboxyl group may include lauric acid and sodium laurate; and the compound containing a hydroxyl group may include ethylene glycol, glycerol, and the like.
  • hydrophilic organic compound may include organic compounds having a triazine ring such as melamine (Chemical name: 1,3,5-triazine-2,4,6-triamine), isocyanuric acid, and isocyanuric acid condensate.
  • melamine Chemical name: 1,3,5-triazine-2,4,6-triamine
  • isocyanuric acid and isocyanuric acid condensate.
  • the hydrophilic compounds may be used alone or as a mixture of two or more kinds.
  • the hydrophilic polymer refers to a polymer having a moisture absorption ratio, measured in accordance with ASTM D 570, of 0.5% by weight or more, and may include generally called hygroscopic polymers, water-absorbing polymers which are not dissolved in water, absorb water in an amount of several times to several hundred times of the polymer, and are not easily dehydrated even if a pressure is applied thereto, and water-soluble polymers which are dissolved in water at an ordinary temperature to a high temperature.
  • hydrophilic polymer may include:
  • the nonionic water-absorbing polymer and the polyether-polyolefin resin block copolymer are preferable because they have comparatively good dispersion stability in a pressure resistant vessel and exhibit the water-absorbability by a comparatively small amount of addition.
  • hydrophilic compounds glycerol, polyethylene glycol, propylene glycol, and melamine are preferable, because a foamed particle having a high expansion ratio can be easily obtained even if an inorganic gas is used as the foaming agent, and a foaming pressure for obtaining a foamed particle having a desired expansion ratio can be reduced, thus resulting in the reduced pressure resistance of facilities, which leads to cost reduction.
  • the polyethylene resin foamed particles of the present invention contain the hydrophilic compound in an amount of preferably 0.01 parts by weight or more and 10 parts by weight or less, more preferably 0.03 parts by weight or more and 5 parts by weight or less, even more preferably 0.05 parts by weight or more and 1 part by weight or less, based on 100 parts by weight of the polyethylene resin mixture (the base resin).
  • the hydrophilic compound is contained in an amount of less than 0.01 parts by weight, an effect of obtaining a foamed particle having a high expansion ratio does not tend to be obtained.
  • it is more than 10 parts by weight, it tends to be difficult to exhibit an effect of further improving the expansion ratio, and it is concerned that the surface beauty and the mechanical properties of the obtained in-mold foam-molded article are impaired.
  • the polyethylene resin foamed particles of the present invention can be produced.
  • the preferable method for producing the polyethylene resin foamed particles may include a method for producing the polyethylene resin foamed particles in an aqueous dispersion system in which the polyethylene resin particles are dispersed in an aqueous dispersion medium together with a foaming agent in a sealed vessel, the mixture is heated to a temperature equal to or higher than a softening temperature of the polyethylene resin particles and pressurized them, and then a foaming step is performed in which the polyethylene resin particles impregnated with the foaming agent are released into an area having a pressure lower than an inner pressure of a sealed vessel (in usual, the atmospheric pressure), thereby obtaining polyethylene resin foamed particles.
  • the polyethylene resin particles, an aqueous dispersion medium and, if necessary, a dispersing agent are filled in a sealed vessel; then, the sealed vessel is subjected to vacuum drawing, if necessary; after that, a foaming agent is introduced into the sealed vessel; and then the vessel is heated to a temperature equal to or higher than a softening temperature of the polyethylene resin.
  • the addition amount of the foaming agent is adjusted so that the inner pressure of the sealed vessel is increased to about 1.5 MPa (gauge pressure) or more and 5 MPa or less (gauge pressure) by heating.
  • the foaming agent is further added to adjust the foaming pressure to a desired value if necessary, and then the content is held for longer than 0 minutes and equal to or shorter than 120 minutes, while the foaming temperature is finely adjusted, and then the content is released into an area having a pressure lower than an inner pressure of a sealed vessel (in usual, the atmospheric pressure), thereby obtaining the polyethylene resin foamed particles.
  • the temperature of the area to which the content is released may be adjusted to room temperature to about 110°C for adjusting the expansion ratio.
  • the method of introducing the foaming agent may be methods other than the method described above, and, for example, a method may be performed in which after the polyethylene resin particles, the aqueous dispersion medium and, if necessary, the dispersing agent are filled in the sealed vessel and then the sealed vessel is subjected to vacuum drawing if necessary, the foaming agent is introduced while the content is heated to a temperature equal to or higher than the softening temperature of the polyethylene resin.
  • a method may be performed in which the polyethylene resin particles, the aqueous dispersion medium and, if necessary, the dispersing agent are filled in the sealed vessel, and then the content is heated to a temperature close to the foaming temperature, and at this time the foaming agent is introduced.
  • the expansion ratio or the average cell diameter of the polyethylene resin foamed particles can be adjusted by injecting carbon dioxide gas, nitrogen, air, a substance which is used as a foaming agent, or the like before the content is released to a low-pressure area to increase the inner pressure of the sealed vessel and adjust a pressure release speed upon foaming, and further by introducing the carbon dioxide gas, nitrogen, air, the substance which is used as the foaming agent, or the like into the sealed vessel during the release to the low-pressure area to control the pressure.
  • the expansion ratio and the average cell diameter by appropriately changing the inner temperature of the sealed vessel before the release to the low-pressure area (approximately a foaming temperature).
  • the expansion ratio of the polyethylene resin foamed particles tends to be increased by increasing the inner pressure of the sealed vessel, increasing the pressure release speed, rising the inner temperature of the sealed vessel before the release, or the like.
  • the average cell diameter of the polyethylene resin foamed particle tends to be decreased by increasing the inner pressure of the sealed vessel, increasing the pressure release speed, or the like.
  • the foaming agent used in the present invention may include saturated hydrocarbons such as propane, butane, and pentane; ethers such as dimethyl ether; alcohols such as methanol and ethanol; inorganic gas such as air, nitrogen, and carbon dioxide gas; and water.
  • the foaming agents may be used alone or as a mixture of multiple kinds. Of these foaming agents, it is particularly preferable to use carbon dioxide gas, nitrogen, air, or water, because of the small environmental burden and no danger of combustion, and carbon dioxide gas is most preferable because a foamed particle having a comparatively high expansion ratio can be easily obtained.
  • the polyethylene resin foamed particles of the present invention show the two melting peaks of the melting peak on a low temperature side and the melting peak on a high temperature side on the DSC curve obtained by the differential scanning calorimetry (DSC).
  • DSC differential scanning calorimetry
  • the DSC curve obtained by the differential scanning calorimetry of the polyethylene resin foamed particles refers to a DSC curve obtained when 1 mg or more and 10 mg or less of the polyethylene resin foamed particles are heated at a temperature rising speed of 10°C/minute from 20°C to 220°C by using a differential scanning calorimeter.
  • the peak temperature difference is preferably 5°C or higher. When it is lower than 5°C, the moldability may possibly be worsened even if the polyethylene resin particles of the present invention are used.
  • the two melting peaks of the melting peak on a low temperature side and the melting peak on a high temperature side can be obtained on the DSC curve.
  • a quantity of heat (Q1) of the melting peak on a low temperature side and a quantity of heat (Qh) of the melting peak on a high temperature side are defined as follows:
  • A a point at which an endothermic amount is the smallest between the two melting points of the melting peak on a low temperature side and the melting peak on a high temperature side on the DSC curve
  • B a tangent point on the high temperature side
  • C a tangent point on the low temperature side
  • a part surrounded by a line AB and the DSC curve is referred to as a quantity of heat (Qh) of the melting peak on a high temperature side
  • a part surrounded by a line AC and the DSC curve is referred to as a quantity of heat (Ql) of the melting peak on a low temperature side.
  • DSC ratio the quantity of heat of the melting peak on a high temperature side
  • DSC ratio the quantity of heat (Qh) of the melting peak on a high temperature side to the total quantity of heat of melting peaks
  • the polyethylene resin foamed particles of the present invention have a DSC ratio of preferably 4% or more and 50% or less, more preferably 7% or more and 35% or less.
  • the expandability of the polyethylene resin foamed particles is too high, and only foamed particles existing near to a mold surface (a part of a surface layer of the in-mold foam-molded article) are foamed to fuse the foamed particles to each other at the initial stage of the in-mold foam-molding; as a result, steam, which is used for the in-mold foam-molding, is not permeated into the inside of the foamed particle, and the obtained article tends to be an in-mold foam-molded article with poor fusion in which the inside of the in-mold foam-molded article is not fused.
  • the open cell ratio of the molded article tends to be increased.
  • the polyethylene resin foamed particles have a DSC ratio of more than 50%, the expandability of the polyethylene resin foamed particles is too low, and there is a tendency that the whole of the in-mold foam-molded article is poor in the fusion or has the poor appearance, or a high molding pressure is necessary for the fusion.
  • the DSC curve in the polyethylene resin foamed particles of the present invention depends on the property of the base resin, and it is preferable that the DSC ratio is 0.05 times or more and 0.4 times or less of the mixing ratio (%) of the polyethylene resins (A and A') in the base resin.
  • the DSC ratio is within the range described above, the open cell ratio is low, and an in-mold foam-molded article having a good surface beauty can be easily obtained.
  • the DSC ratio in the polyethylene resin foamed particles by appropriately changing the aforementioned temperature inside the sealed vessel (hereinafter which may sometimes be referred to as "foaming temperature") or the holding time before the release to the low pressure area, when the polyethylene resin foamed particles are obtained.
  • the DSC ratio tends to be increased by lowering the temperature inside the sealed vessel (the foaming temperature), prolonging the holding time, or the like.
  • the DSC ratio tends to be decreased by raising the temperature inside the sealed vessel (the foaming temperature), shortening the holding time, or the like.
  • the sealed vessel used in the present invention is not particularly limited, and any vessel can be used so long as it can withstand a pressure inside the vessel and a temperature inside the vessel when the foamed particles are produced. Examples thereof may include an autoclave pressure resistant vessel.
  • aqueous dispersion medium used in the present invention it is preferable to use only water, however, a dispersion medium in which methanol, ethanol, ethylene glycol, or glycerol is added to water may be used.
  • a dispersion medium in which methanol, ethanol, ethylene glycol, or glycerol is added to water may be used.
  • water in the aqueous dispersion medium also acts as the foaming agent, and contributes to the improvement of the expansion ratio.
  • a dispersing agent in an aqueous dispersion medium in order to prevent cohesion of the polyethylene resin particles to each other.
  • the dispersing agent used in the present invention may include inorganic dispersing agents such as tribasic calcium phosphate, tribasic magnesium phosphate, basic magnesium carbonate, calcium carbonate, barium sulfate, kaolin, talc, and clay.
  • the dispersing agents may be used alone or as a mixture of two or more kinds.
  • Examples of the dispersing aid, used in the present invention may include anionic surfactants such as carboxylate surfactants including an N-acylamino acid salt, an alkyl ether carboxylate, and acylated peptide; sulfonate surfactants including an alkyl sulfonate, an n-paraffin sulfonate, an alkyl benzene sulfonate, an alkyl naphthalene sulfonate, and a sulfosuccinate; sulfate surfactants including sulfated oil, an alkyl sulfate, an alkyl ether sulfate, an alkyl amide sulfate, and an alkyl allyl ether sulfate; phosphate surfactants including an alkyl phosphate, a polyoxyethylene phosphate, and the like.
  • anionic surfactants such as carboxylate surfactants including an N-acylamino acid
  • the dispersing aid it is possible to use polycarboxylic acid polymer surfactants such as a maleic acid copolymer salt and a polyacrylate; and polyvalent anionic polymer surfactants such as a polystyrene sulfonate, and a formalin condensate salt with naphthalenesulfonic acid.
  • the dispersing aids may be used alone or as a mixture of two or more kinds.
  • At least one compound selected from the group consisting of tribasic calcium phosphate, tribasic magnesium phosphate, barium sulfate, and kaolin as the dispersing agent together with at least one compound selected from the group consisting of sodium n-paraffin sulfonate and alkyl benzene sulfonic acid as the dispersing aid.
  • the amount of the dispersing agent or the dispersing aid used in the present invention varies depending on the kind thereof, and the kind and amount of the polyethylene resin particles used, and in usual, it is preferable to use the dispersing agent in an amount of 0.1 parts by weight or more and 3 parts by weight or less and the dispersing aid in an amount of 0.001 parts by weight or more and 0.1 parts by weight or less, based on 100 parts by weight of the aqueous dispersion medium. In order to improve the dispersibility of the polyethylene resin particles in the aqueous dispersion medium, it is preferable to use the polyethylene resin particles in an amount of 20 parts by weight or more and 100 parts by weight or less, based on 100 parts by weight of the aqueous dispersion medium.
  • the polyethylene resin foamed particles in the aqueous dispersion system described above, it is also possible to obtain the polyethylene resin foamed particles by bringing the polyethylene resin particles into direct contact with the foaming agent in the sealed vessel, without using the aqueous dispersion medium, to impregnate the particles with the foaming agent and to obtain foamable polyethylene resin particles, and then bringing the foamable polyethylene resin particles into contact with steam to foam them.
  • a step in which the polyethylene resin foamed particles are obtained from the polyethylene resin particles, as described above, may sometimes be referred to as "first stage foaming step” and the thus obtained polyethylene resin foamed particles may sometimes be referred to as “first stage foamed particles.”
  • the first stage foamed particles are further impregnated with inorganic gas (for example, air, nitrogen, carbon dioxide gas, or the like) to provide an inner pressure to them, and then the particles are brought into contact with steam having a specific pressure, the polyethylene resin foamed particles having an expansion ratio which is higher than that of the first stage foamed particles can be obtained.
  • a step in which the polyethylene resin foamed particles are further foamed to provide polyethylene resin foamed particles having a higher expansion ratio, as described above, may sometimes be referred to as "second stage foaming step” and polyethylene resin foamed particles obtained through the second stage foaming step may sometimes be referred to as "second stage foamed particles.”
  • the "second stage foaming step” refers specifically to a step in which the first stage foamed particles are impregnated with the inorganic gas (for example, air, nitrogen, carbon dioxide gas, and the like) to provide the inner pressure to them, and then the resulting particles are brought into contact with steam having a specific pressure to obtain second stage foamed particles having an expansion ratio which is higher than that of the first stage foamed particles.
  • the inorganic gas for example, air, nitrogen, carbon dioxide gas, and the like
  • the steam pressure in the second stage foaming step is adjusted to a range of 0.02 MPa (gauge pressure) or more and 0.15 MPa (gauge pressure) or less, more preferably 0.03 MPa (gauge pressure) or more and 0.1 MPa (gauge pressure) or less, considering the expansion ratio of the second stage foamed particles.
  • the inner pressure of the inorganic gas with which the first stage foamed particles are impregnated is appropriately changed considering the expansion ratio and the like of the second stage foamed particles, however, it is preferably 0.12 MPa or more (absolute pressure) and 0.6 MPa or less (absolute pressure).
  • the expansion ratio of the polyethylene resin foamed particles of the present invention is not particularly limited, and may be adjusted as necessary.
  • the expansion ratio of the polyethylene resin foamed particles is preferably 2 times or more and 50 times or less, more preferably 8 times or more and 45 times or less, even more preferably 11 times or more and 40 times or less, in terms of the weight reduction and the mechanical properties of the polyethylene resin in-mold foam-molded article.
  • the expansion ratio of the polyethylene resin foamed particles is less than 2 times, the effect of the weight reduction is small, and when it is more than 50 times, the mechanical properties such as a compressive stress of the polyethylene resin in-mold foam-molded article tend to be decreased.
  • the polyethylene resin foamed particles of the present invention preferably have an average cell diameter of 80 ⁇ m or more and 500 ⁇ m or less, more preferably 120 ⁇ m or more and 400 ⁇ m or less.
  • the polyethylene resin foamed particles have an average cell diameter of less than 80 ⁇ m, the obtained polyethylene resin in-mold foam-molded article tends to be greatly shrunk, and when it is more than 500 ⁇ m, the appearance of the obtained polyethylene resin in-mold foam-molded article tends to be worsened.
  • the average cell diameter is measured as follows: In an image obtained by an observation with a microscope of a cross-section at a center of the foamed particle, a straight line, passing through the almost center of the foamed particle, is drawn and the number n of cells through which the straight line is passed and a foamed particle diameter L ( ⁇ m), decided by an intersection point of the straight line and the foamed particle surface, are read.
  • the polyethylene resin foamed particles of the present invention have preferably an open cell ratio of 10% or less, more preferably 5% or less.
  • the polyethylene resin foamed particles have an open cell ratio of more than 10%, shrinkage is caused upon the in-mold foam-molding, and the surface property of the obtained polyethylene resin in-mold foam-molded article tends to be reduced and the compressive strength also tends to be reduced.
  • the polyethylene resin foamed particle obtained as above are subjected to an in-mold foam-molding in which the particles are filled in a mold having a pre-determined shape and are heated with steam or the like to fuse the foamed particles to each other, whereby a polyethylene resin in-mold foam-molded article can be obtained.
  • the method (c) which is the simplest method, is desirably used, because a molded article having a beautiful appearance and a small dimensional shrinkage percentage to mold can be obtained even if the method (c) is used.
  • a specific method of in-mold foam-molding a polyethylene resin in-mold foam-molded article from the polyethylene resin foamed particles of the present invention may include, for example, a method in which the polyethylene resin foamed particles, which has not been previously subjected to a pre-treatment, are filled in a molding space formed of two molds and capable of being closed but incapable of being sealed, and molded at a heating steam pressure of about 0.05 to 0.20 MPa (gauge pressure) for a heating time of about 3 to 30 seconds using a heating medium such as steam to fuse the polyethylene resin foamed particles to each other, the mold is cooled with water and then opened, thereby obtaining a polyethylene resin in-mold foam-molded article.
  • Polyethylene resins used in Examples and Comparative Examples were purchased from polyolefin makers [SK Chemicals Co., Ltd., Japan Polyethylene Corporation, The Dow Chemical Company, Prime Polymer Co., Ltd., Ube-Maruzen Polyethylene Co., Ltd., and the like]. Physical properties of each polyethylene resin are shown in Table 1.
  • a flat plate-like space of 100 mm (length) ⁇ 100 mm (width) was formed using spacers having a thickness of 1.0 mm in a press machine [37 TS Molding Machine manufactured by Fuji Setsubi Kogyo Kabushiki Kaisha], which had been previously pre-heated at 190°C, and polyethylene resin particles were hot pressed with a pressure of 56kg/cm 2 (5.5 MPa) for 5 minutes. After that, the resin particles were cooled in the pressed state until the temperature reached 50°C and then the spacers were taken away to obtain a polyethylene resin plate having a length of about 100 mm, a width of about 100 mm and a thickness of about 1 mm. The obtained polyethylene resin plate was punched using a punch having a diameter of ⁇ 25 mm to obtain test pieces.
  • a viscoelasticity measuring apparatus As a measurement apparatus, a viscoelasticity measuring apparatus, ARES, manufactured by TA Instruments Inc., was used, with which a cone plate-type tool having a diameter of 25 mm and a cone angle of 0.04 rad was loaded.
  • a thermostatic chamber was set so that it enclosed the tool, the tool is kept at 190°C, the thermostatic chamber was opened after the tool was pre-heated, a test piece was inserted into parallel plates, the thermostatic chamber was closed, it was pre-heated for 5 minutes, and compression was performed until a space between the cone plates reached 0.07 mm as described in a calibration certificate. After the compression, the thermostatic chamber was opened again, the resin protruding from the cone plates was scraped up by a brass spatula, the thermostatic chamber was closed, it was kept hot for 5 minutes again, and then the dynamic viscoelasticity measurement was started.
  • the measurement was performed in a range of an angular frequency of 0.1 rad/second to 100 rad/second, and a storage elastic modulus and a loss elastic modulus were obtained at each angular frequency.
  • values of the storage elastic modulus at angular frequencies of 1 rad/second and 100 rad/second were adopted.
  • the strain amount was 5% and the measurement was performed in a nitrogen atmosphere.
  • the temperature of 5 to 6 mg of the obtained polyethylene resin particles was raised at a temperature rising speed of 10°C/minute from 20°C to 220°C, lowered at a speed of 10°C/minute to 10°C, and then raised again at a temperature rising speed of 10°C/minute to 220°C, thereby obtaining a DSC curve on a second temperature rising (an example thereof is shown in Fig. 2 ).
  • a total quantity of heat (q) of the endothermic peaks on the obtained DSC curve was referred to as a crystal melting heat of the resin particles.
  • An MI of the polyethylene resin particles was measured using an MI measuring instrument described in JIS K 7210, under conditions of an orifice diameter of 2.0959 ⁇ 0.005 mm ⁇ , an orifice length of 8.000 ⁇ 0.025 mm, a load of 2160 g, and a temperature of 190 ⁇ 0.2°C.
  • the obtained polyethylene resin first stage foamed particles were observed and the evaluation was performed by the following criteria:
  • the expansion ratio was calculated as a true specific gravity ratio (pr/pb) of the true specific gravities before and after the foaming.
  • the obtained polyethylene resin second stage foamed particles were cut at the center of the foamed particle using a double blade razor ["Hi Stainless” double blade manufactured by Feather Safety Razor Co., Ltd.].
  • a DSC curve was obtained when the temperature of 5 to 6 mg of the obtained polyethylene resin first stage foamed particles was raised at a temperature rising speed of 10°C/minute from 20°C to 220°C, using a differential scanning calorimeter [DSC 6200 manufactured by Seiko Instruments Inc.] (an example thereof is shown in Fig. 1 ).
  • a temperature difference (Th-Tl) between a temperature (Tl) at the melting peak on a low temperature side and a temperature (Th) at the melting peak on a high temperature side on the obtained DSC curve was referred to as a melting peak temperature difference.
  • a volume Vc (cm 3 ) of the obtained polyethylene resin second stage foamed particles was measured using an air-comparison pycnometer [Model 1000 manufactured by Tokyoscience Co., Ltd.] according to a method described in ASTM D 2856-87 PROCEDURE C.
  • Open Cell Ratio % Va ⁇ Vc ⁇ Va ⁇ 100
  • each foam-molded article to be evaluated which is obtained by molding the particles in a manner in which a pre-determined steam pressure was changed 0.01 MPa by 0.01 MPa within a range of 0.10 to 0.18 MPa (gauge pressure) in the main heating step in [Production of Polyethylene Resin In-Mold Foam-Molded Article]
  • a crack having a depth of about 5 mm was made in the surface of the in-mold foam-molded article with a knife, the article was fractured along with the crack, the fracture surface was observed, a ratio of the number of fractured particles to the number of total particles on the fracture surface was obtained, and a fusion ratio of the molded article was evaluated.
  • the minimum steam pressure at which the fusion ratio reached 80% or more was referred to as the minimum molding pressure.
  • a volume vc (cm 3 ) of the foam sample for measurement was measured using an air-comparison pycnometer [Model 1000 manufactured by Tokyoscience Co., Ltd.] according to a method described in ASTM D 2856-87 PROCEDURE C.
  • an apparent volume va (cm 3 ) of the polyethylene resin foam was measured by sinking the whole polyethylene resin foam sample after the vc measurement in ethanol in a measuring cylinder, and reading a liquid level rising in the measuring cylinder (a submersion method).
  • Open Cell Ratio % of Foam-Molded Article va ⁇ vc ⁇ va ⁇ 100
  • the maximum molding pressure was defined as the maximum pressure at which the open cell ratio reached 22% or less. There was a tendency in which the in-mold foam-molded article was greatly shrunk when the open cell ratio is more than 20 to 25%.
  • the difference between the minimum molding pressure and the maximum molding pressure was defined as the mold-processing range.
  • the edge of the foam-molded article refers to a ridge line section at which the surfaces of the in-mold foam-molded article intersect with one another.
  • Adjacent foamed particles are finely fused to each other at any part, and there are no spaces between the foamed particles.
  • a longitudinal size (a direction of 400 mm) of foam-molded article molded in a condition of the minimum molding pressure + 0.01 MPa among the obtained foam-molded articles to be evaluated was measured using a digital caliper [manufactured by Mitutoyo Corporation].
  • a dimensional shrinkage percentage to mold was calculated by the following formula in which L 0 is a size of a corresponding mold, and L 1 is a size of a foam-molded article, and the evaluation was made by the following criteria.
  • Dimensional Shrinkage Percentage to Mold % L 0 ⁇ L 1 ⁇ L 0 ⁇ 100
  • an open cell ratio (%) of the foam-molded article was calculated according to the method described in " ⁇ Maximum Molding Pressure on Molding>.”
  • a linear polyethylene resin 90 parts by weight of the component (A-1) and 10 parts by weight of the component (B-1) were mixed, with which 0.2 parts by weight of glycerol and 0.1 parts by weight of talc as a cell-nucleating agent, based on 100 parts by weight of the total amount of the mixed linear polyethylene resin, were dry-blended.
  • the dry-blended mixture was thrown into a twin-screw extruder having a bore diameter of 45 mm, melt-kneaded at a resin temperature of about 220°C, and extruded through a circular die equipped on a tip of the extruder into strands, and they were cooled with water and cut with a cutter to obtain polyethylene resin particles.
  • the particles had a weight per particle of 4.5 mg.
  • the evaluation results of the resin pressure upon the production are shown in Table 2.
  • a pressure-resistant autoclave having a volume of 10 L were filled 100 parts by weight (2.4 kg) of the obtained polyethylene resin particles, 200 parts by weight of water, 0.5 parts by weight of tribasic calcium phosphate, which was a hardly water-soluble inorganic compound, and 0.03 parts by weight of an sodium alkylsulfonate (sodium n-paraffinsulfonate), which was a surfactant, and then 7 parts by weight of carbon dioxide gas was added thereto as a foaming agent with stirring.
  • sodium alkylsulfonate sodium n-paraffinsulfonate
  • the temperature of the content in the autoclave was raised to a foaming temperature of 121.6°C described in Table 2.
  • the carbon dioxide gas was additionally introduced with pressure into the autoclave, to increase the autoclave inner pressure to a foaming pressure of 3.5 MPa-G described in Table 2.
  • a valve located on the lower part of the autoclave was opened to release the content in the autoclave through an open orifice (one hole) having a diameter of 4.0 mm into an atmosphere having a temperature of 100°C, thereby obtaining polyethylene resin foamed particles.
  • the obtained polyethylene resin second stage foamed particles After moisture was removed from the obtained polyethylene resin second stage foamed particles, they were filled in a mold having a flat plate-like molding space having a length of 400 mm, a width of 300 mm, and a thickness of 60 mm, and the inside of the mold chamber was heated with steam for 10 seconds. After that, an exhaust valve was closed, and the mold was heated with steam for 12 seconds to fuse the foamed particles to each other. Subsequently, after the steam was exhausted and the inside of the mold and the surface of the molded article were cooled with water, the molded article was taken out to obtain a polyethylene resin in-mold foam-molded article.
  • the pre-determined steam pressure was changed 0.01 MPa by 0.01 MPa within a range of 0.10 to 0.18 MPa (gauge pressure) and molding was performed at each steam pressure.
  • the holding time at the pre-determined pressure was 4 seconds.
  • Each of the obtained foam-molded article was allowed to stand at 23°C for 2 hours, and then it was aged at 75°C for 24 hours, and after that it was allowed to stand in a room having a temperature of 23°C for 4 hours, whereby a subject to be evaluated was obtained.
  • the minimum value of the steam pressure (the minimum molding pressure) at which the fusion ratio reached 80% or more was specified, the dimensional shrinkage percentage to mold and the surface beauty of the foam-molded article, which was molded in a condition of the minimum molding pressure + 0.01 MPa were evaluated.
  • the open cell ratio the open cell ratio (L) obtained in the minimum molding pressure and the open cell ratio (H) obtained in the condition of the minimum molding pressure + 0.02 MPa were measured. The results are shown in Table 2.
  • Polyethylene resin particles, polyethylene resin foamed particles, and a polyethylene resin in-mold foam-molded article were produced in the same manner as in Example 1, except that in [Production of Polyethylene Resin Particle], the kind and the mixing amount of the linear polyethylene resin were changed as shown in Table 2, and in [Production of Polyethylene Resin Foamed particles], the foaming temperature and foaming pressure on the first stage foaming and the inner pressure and steam pressure on the second stage foaming were changed as shown in Table 2.
  • Polyethylene resin particles, polyethylene resin foamed particles, and a polyethylene resin in-mold foam-molded article were produced in the same manner as in Example 1, except that in [Production of Polyethylene Resin Particles], glycerol was not added, and in [Production of Polyethylene Resin Foamed particles], the foaming temperature and the foaming pressure on the first stage foaming and the inner pressure and the steam pressure on the second stage foaming were changed as shown in Table 2.
  • Polyethylene resin particles, polyethylene resin foamed particles, and a polyethylene resin in-mold foam-molded article were produced in the same manner as in Example 1, except that in [Production of Polyethylene Resin Particles], the kind and the mixing amount of the linear polyethylene resin were changed as shown in Table 3, and in [Production of Polyethylene Resin Foamed particles], the foaming temperature and the foaming pressure on the first stage foaming, and the inner pressure and the steam pressure on the second stage foaming were changed as shown in Table 3.
  • Polyethylene resin foamed particles were produced in the same manner as in Example 1 except that in ⁇ First stage Foaming> of [Production of Polyethylene Resin Foamed particles], the foaming temperature on the first stage foaming was changed to 130°C.
  • the obtained first stage foamed particles had many wrinkles, which were greatly shrunk beads, and had one melting peak on the DSC curve of the foamed particles.
  • the ⁇ second stage foaming> was performed in the same manner as in [Production of Polyethylene Resin Foamed particles] of Example 1, however, the foamed particles were not foamed, and the foamed particle having a desired expansion ratio could not obtained.
  • the obtained molded article has a low open cell ratio of 20% or less, and even if the steam pressure is increased by 0.02 MPa from the minimum molding pressure, the increase of the open cell ratio of the molded article was 5% or less, and the in-mold foam-molded article having the good appearance could be obtained. Even if the molding steam pressure is changed, the good in-mold foam-molded article can be obtained by using the foamed particles of the present invention, and thus it was possible to improve the production stability and to stabilize the quality. Furthermore, the resin pressure, which is an indicator of the productivity on the resin particle production, was 8.0 MPa or less, and the productivity of the resin particles was good.
  • Example 9 From the comparison of Example 1 with Example 9, it is seen that the presence of glycerol, which is the hydrophilic substance, can suppress the foaming pressure.
  • a polyethylene resin 80 parts by weight of the component (A-3) and 20 parts by weight of the component (B-1) were mixed, with which 0.2 parts by weight of glycerol and 0.1 parts by weight of talc as a cell-nucleating agent, based on 100 parts by weight of the total amount of the mixed linear polyethylene resin, were dry-blended.
  • the dry-blended mixture was thrown into a twin-screw extruder having a bore diameter of 45 mm, melt-kneaded at a resin temperature of about 220°C, and extruded through a circular die equipped on a tip of the extruder into strands, and they were cooled with water and cut with a cutter to obtain polyethylene resin particles.
  • the particles had a weight per particle of 4.5 mg.
  • a pressure-resistant autoclave having a volume of 10 L were filled 100 parts by weight (2.4 kg) of the obtained polyethylene resin particles, 200 parts by weight of water, 0.5 parts by weight of tribasic calcium phosphate, which was a hardly water-soluble inorganic compound, and 0.03 parts by weight of an sodium alkylsulfonate (sodium n-paraffinsulfonate), which was a surfactant, and then 7 parts by weight of carbon dioxide gas was added thereto as a foaming agent with stirring.
  • sodium alkylsulfonate sodium n-paraffinsulfonate
  • the temperature of the content in the autoclave was raised to a foaming temperature of 122.4°C described in Table 4.
  • the carbon dioxide gas was additionally introduced with pressure into the autoclave, to increase the autoclave inner pressure to a foaming pressure of 3.5 MPa-G described in Table 4.
  • a valve located on the lower part of the autoclave was opened to release the content in the autoclave through an open orifice (one hole) having a diameter of 4.0 mm into an atmosphere having a temperature of 100°C, thereby obtaining polyethylene resin foamed particles.
  • the particles were filled in the pressure resistant vessel and impregnated with air by pressurization, whereby an inner pressure of the first stage foamed particles was adjusted to 0.20 MPa.
  • a second stage foaming was performed by heating the particles with steam (a steam pressure of 0.053 MPa-G), thereby obtaining polyethylene resin second stage foamed particles having an expansion ratio of about 25 times.
  • the obtained polyethylene resin second stage foamed particles After moisture was removed from the obtained polyethylene resin second stage foamed particles, they were filled in a mold having a flat plate-like molding space having a length of 400 mm, a width of 300 mm, and a thickness of 60 mm, and the inside of the mold chamber was heated with steam for 10 seconds. After that, an exhaust valve was closed, and the mold was heated with steam for 12 seconds to fuse the foamed particles to each other. Subsequently, after the steam was exhausted and the inside of the mold and the surface of the molded article were cooled with water, the molded article was taken out to obtain a polyethylene resin in-mold foam-molded article.
  • the pre-determined steam pressure was changed 0.01 MPa by 0.01 MPa within a range of 0.10 to 0.18 MPa (gauge pressure) and molding was performed at each steam pressure.
  • the holding time at the pre-determined pressure was 4 seconds.
  • Each of the obtained foam-molded article was allowed to stand at 23°C for 2 hours, and then it was aged at 75°C for 24 hours, and after that it was allowed to stand in a room having a temperature of 23°C for 4 hours, whereby a subject to be evaluated was obtained.
  • the minimum value of the steam pressure (the minimum molding pressure) at which the fusion ratio reached 80% or more was specified, the dimensional shrinkage percentage to mold and the surface beauty of the foam-molded article, which was molded in a condition of the minimum molding pressure + 0.01 MPa were evaluated.
  • the evaluation of the open cell ratio of the obtained in-mold foam-molded article was measured, and the maximum molding pressure was specified. The results are shown in Table 4.
  • Polyethylene resin particles, polyethylene resin foamed particles, and a polyethylene resin in-mold foam-molded article were produced in the same manner as in Example 10, except that in [Production of Polyethylene Resin Particles], the kind and mixing amount of the polyethylene resin were changed as shown in Table 4, and in [Production of Polyethylene Resin Foamed particles], the foaming temperature and the foaming pressure on the first stage foaming and the inner pressure and the steam pressure on the second stage foaming were changed as shown in Table 4, with Examples 13 and 14 being Reference Examples.
  • Polyethylene resin particles, polyethylene resin foamed particles, and a polyethylene resin in-mold foam-molded article were produced in the same manner as in Example 10, except that in [Production of Polyethylene Resin Particles], the kind and the mixing amount of the polyethylene resin were changed as shown in Table 5, and in [Production of Polyethylene Resin Foamed particles], the foaming temperature and the foaming pressure on the first stage foaming, and the inner pressure and the steam pressure on the second stage foaming were changed as shown in Table 5.
  • the resin pressure was 8.0 MPa or less on the resin particle production.
  • the in-mold foam-molded articles having the good appearance in a broad mold-processing range were obtained, in spite of a narrow melting peak temperature difference of the foamed particles of 11°C or lower.
  • the foamed particles of the present invention are used, even if the polyethylene resin foamed particles having a narrow melting peak temperature difference, such as polyethylene resins having a high degree of crystallinity, are used, the productivity is good, and the moldability in the in-mold foam-molding is good.

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Claims (12)

  1. Particules expansées de résine de polyéthylène, obtenues par expansion de particules de résine de polyéthylène dans lesquelles une résine de polyéthylène linéaire est une résine de base,
    dans lesquelles le module élastique au stockage est de 900 Pa ou plus et de 5000 Pa ou moins à une fréquence angulaire de 1 rad/seconde et le module élastique au stockage est de 100 000 Pa ou moins à une fréquence angulaire de 100 rad/seconde dans une mesure de viscoélasticité dynamique des particules de résine de polyéthylène à 190°C, comme davantage spécifié dans la description ; et
    la courbe de DSC pour les particules expansées de résine de polyéthylène, obtenue par calorimétrie à balayage différentiel (DSC) à une vitesse de montée en température de 10°C/min dans la plage allant de 20°C à 220°C a deux pics de fusion consistant en un pic de fusion du côté des basses températures et un pic de fusion du côté des températures élevées.
  2. Particules expansées de résine de polyéthylène selon la revendication 1, dans lesquelles la résine de base est une résine mixte contenant (A) une résine de polyéthylène linéaire ayant un indice de fusion de 1,2 g/10 minutes ou plus et de 10 g/10 minutes ou moins en une proportion de 60 % en poids ou plus et de 97 % en poids ou moins, et (B) une résine de polyéthylène linéaire ayant un indice de fusion de 0,01 g/10 minutes ou plus et de 0,3 g/10 minutes ou moins en une proportion de 3 % en poids ou plus et de 40 % en poids ou moins, le total de (A) et (B) étant de 100 % en poids, et
    les particules de résine de polyéthylène ont un indice de fusion de 0,8 g/10 minutes ou plus et de 3,0 g/10 minutes ou moins, et
    dans lesquelles l'indice de fusion est mesuré à une température de 190°C avec une charge de 2,16 kg conformément à la norme JIS K 7210.
  3. Particules expansées de résine de polyéthylène selon la revendication 1, dans lesquelles la différence entre les températures respectives aux deux pics de fusion est de 11°C ou moins.
  4. Particules expansées de résine de polyéthylène selon la revendication 3, dans lesquelles, quand la température est augmentée à une vitesse de montée en température de 10°C/minutes de 20°C à 220°C par utilisation d'un calorimètre à balayage différentiel (DSC), abaissée à 10°C à une vitesse de 10°C/minute, et de nouveau augmentée à une vitesse de montée en température de 10°C/minute jusqu'à 220°C,
    les particules de résine ont une chaleur de fusion cristalline q de 145 J/g ou plus, la chaleur de fusion cristalline q étant calculée à partir d'une courbe de DSC obtenue durant la deuxième montée en température.
  5. Particules expansées de résine de polyéthylène selon la revendication 3 ou 4, dans lesquelles une résine de base des particules de résine de polyéthylène est une résine mixte de (A') une résine de polyéthylène ayant un indice de fusion de 1,2 g/10 minutes ou plus et de 10 g/10 minutes ou moins en une proportion de 60 % en poids ou plus et de 97 % en poids ou moins, et (B') une résine de polyéthylène ayant un indice de fusion de 0,01 g/10 minutes ou plus et de 0,3 g/10 minutes ou moins en une proportion de 3 % en poids ou plus et de 40 % en poids ou moins, le total de (A') et (B') étant de 100 % en poids, et
    les particules de résine de polyéthylène ont un indice de fusion de 0,8 g/10 minutes ou plus et de 3,0 g/10 minutes ou moins, l'indice de fusion étant mesuré à une température de 190°C avec une charge de 2,16 kg conformément à la norme JIS K 7210.
  6. Particules expansées de résine de polyéthylène selon l'une quelconque des revendications 1 à 5, dans lesquelles un composé hydrophile est contenu en une proportion de 0,01 partie en poids ou plus et de 10 parties en poids ou moins pour 100 parties en poids de la résine de base des particules de résine de polyéthylène.
  7. Particules expansées de résine de polyéthylène selon l'une quelconque des revendications 1 à 6, lesquelles particules expansées ont une proportion d'alvéoles ouverts de 10 % ou moins, la proportion d'alvéoles ouverts étant calculée par la formule : proportion d'alvéoles ouverts % = Va − Vc ÷ Va × 100
    Figure imgb0014
     à partir du volume Vc mesuré conformément à une méthode décrite dans la norme ASTM D 2856-87 PROCEDURE C, utilisant un pycnomètre de comparaison à l'air, et un volume apparent Va mesuré conformément à une méthode en immersion.
  8. Article moulé en mousse dans un moule de résine de polyéthylène obtenu par remplissage d'un moule avec n'importe lesquelles des particules expansées de résine de polyéthylène selon l'une quelconque des revendications 1 à 7, et ensuite moulage de celles-ci en mousse dans le moule.
  9. Méthode pour produire des particules expansées de résine de polyéthylène de l'une quelconque des revendications 1 à 7, comprenant une première étape d'expansion,
    dans laquelle la première étape d'expansion est une étape pour produire des particules expansées de résine de polyéthylène comprenant : la dispersion de particules de résine de polyéthylène dans un milieu de dispersion aqueux conjointement avec un agent moussant ; le chauffage de celles-ci à une température égale ou supérieure à une température de ramollissement des particules de résine de polyéthylène et la pressurisation de celles-ci ; et la libération de celles-ci dans une zone ayant une pression inférieure à la pression interne d'un récipient scellé.
  10. Méthode pour produire des particules expansées de résine de polyéthylène selon la revendication 9, dans laquelle l'agent moussant est un gaz inorganique et/ou l'eau.
  11. Méthode pour produire des particules expansées de résine de polyéthylène selon la revendication 10, dans laquelle le gaz inorganique est le dioxyde de carbone gazeux.
  12. Méthode pour produire un article moulé en mousse dans un moule de résine de polyéthylène comprenant : le remplissage, avec les particules expansées de résine de polyéthylène de l'une quelconque des revendications 1 à 7, d'un espace de moulage comprenant deux moules et étant capable d'être fermé mais incapable d'être scellé, sans mise en œuvre préalable d'un prétraitement ; et le chauffage de celles-ci au moyen d'un milieu chauffant.
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